Pressure-sensitive labels are applied to packages to build brand awareness, show the contents of the package, convey a quality message regarding the contents of a package, and supply consumer information such as directions on product use, or an ingredient listing of the contents. Printing on the pressure-sensitive label is typically done using gravure printing or flexography. There is a continuing need to improve the visual appeal of labels to increase shelf awareness of products. Prior-art printed labels have attempted to provide improved visual information on labels by utilizing multiple print stations in a printing press to achieve “photographic quality.” While nine color presses do provide a good image, thermal-dye transfer systems is an alternative that can potentially provide images having depth, excellent flesh tone replication, excellent tone scale, and superior image sharpness.
Prior-art labels that are applied to packages comprise a base for holding the image and a pressure-sensitive adhesive, previously attached to a liner (carrier). The label media (on which the image is printed) can optionally be in the form of sheets that comprise material for a plurality of labels and are typically made by laminating the necessary single or multi-layer films comprising the media. The images are printed on the label media utilizing a variety of printing methods. After printing, the media surface can be protected by an over-laminate material or a protective coating. Optionally, a plurality of individual labels can be cut into a label media after or before printing and prior to application to packaging or other uses. The completed imaged label consisting of a protection layer, printed information such as an image, base, and pressure-sensitive adhesive, is applied to packages after removing the liner utilizing high-speed labeling equipment.
One method of printing label media is flexography which is an offset letterpress technique where the printing plates are made from rubber or photopolymers. The printing on pressure-sensitive label media is accomplished by the transfer of ink from the raised surface of the printing plate to the surface of the material being printed. The rotogravure method of printing uses a print cylinder with thousands of tiny cells that are below the surface of the printing cylinder. The ink is transferred from the cells when the print cylinder is brought into contact with the pressure-sensitive label media at the impression roll. Printing inks for flexography or rotogravure include solvent-based inks, water-based inks and radiation-cured inks. While rotogravure and flexography printing do provide acceptable image quality, these two printing methods require expensive and time-consuming preparation of print cylinders or printing plates which make printing jobs of less than 100,000 units expensive as the set-up cost and the cost of the cylinders or printing plates is typically depreciated over the size of the print job.
Recently, digital printing has become a viable method for the printing of information on packages. The term “digital printing” refers to the electronic digital characters or electronic digital images that can be printed by an electronic output device capable of translating digital information. The main digital printing technologies are inkjet, electrophotography, and thermal dye transfer.
Digital inkjet printing has the potential to revolutionize the printing industry by making short-run color-print jobs more economical. However, the next commercial stage will require significant improvements in inkjet technology; the major hurdle remaining is to improve print speed. Part of this problem is the limitation of the amount of data the printer can handle rapidly. The more complex the design, the slower the printing process. Right now they are about 10 times slower than comparable digital electrostatic printers.
Another printing technique for labels is disclosed in U.S. Pat. No. 6,566,024 issued May 20, 2003 to Bourdelais et al., titled “Quintessential Pictorial Label And Its Distribution” and involves silver-halide photography. Such printing on label media can provide higher quality images to packaging materials, including the printing of images using an optical digital printing system with the Pantone color space of printed inks.
In recent years, thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta, and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta, or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta, and yellow signals. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are set forth in U.S. Pat. No. 4,621,271 issued Nov. 4, 1986 to Brownstein, titled “Apparatus And Method For Controlling A Thermal Printer Apparatus.”
Thermal-dye-transfer receiving elements (“receivers”) used in thermal-dye-transfer generally comprise a polymeric image-receiving layer coated on a support. Supports are required to have, among other properties, adequate strength, dimensional stability, and heat resistance. For reflective viewing, supports are also desired to be as white as possible. Cellulose paper and plastic films have been proposed for use as dye-receiving element supports in efforts to meet these requirements. Recently, microvoided films formed by stretching an orientable polymer containing an incompatible organic or inorganic material have been suggested for use in thermal dye-transfer receivers.
Thermal-dye-transfer receiving sheets for labels or stickers are known in the art including, for example, U.S. Pat. No. 6,153,558 issued Nov. 28, 2000 to Shirai et al., titled “Thermal Transfer Image-Receiving Sheet For Sticker And Method Of Manufacturing Same;” U.S. Pat. No. 6,162,517 issued Dec. 19, 2000 to Oshima et al., titled “Image-Receiving Sheet For Thermal Transfer Printing;” and U.S. Pat. No. 4,984,823 issued Jan. 15, 1991 to Ishii et al., titled “Label Having Sublimation Transferred Image.” U.S. Pat. No. 6,162,517 issued Dec. 19, 2000 to Oshima et al., titled “Image-Receiving Sheet For Thermal Transfer Printing,” for example, discloses a label comprising, disposed between a dye receptor layer and an adhesive layer, a foamed resin film layer and a non-foamed resin film layer. A bonding layer can be disposed between the foamed and non-foamed layers. U.S. Pat. No. 4,984,823 to Ishii et al. discloses a label portion comprising an image-receiving layer, a sheet substrate, and an adhesive layer. The sheet substrate can be a resin film such as foamed polyethylene terephthalate, synthetic paper, and the like.
There is a continuing need for high-quality labels that can be printed from digital label files that contain graphics, text, and images. Digital printing of labels takes advantage of the growing amount of label data that is resident in digital files. Digital printing, as opposed to the analog flexographic printing of labels, also enables the use of distributive printing of label files, which allows a digital label file to be created in one central location, sent to remote locations, and printed on digital label printers.
Although thermal-dye-transfer receivers for non-label applications have achieved high quality, the construction and nature of thermal dye-transfer media used for making labels are different. For example, labels are usually thinner than the media from which they are made. Thermal-dye transfer media can be more expensive than the media used in other printing techniques, thus making the development of new materials and structural configurations that are competitive for use in high-volume or low cost commercial applications a challenging endeavor.